This invention relates generally to droplet emitters and more particularly concerns an acoustically actuated droplet emitter which is provided with a continuous, high velocity, laminar flow of liquid.
FIG. 1 shows a cross-sectional view of a standard droplet emitter 10 for an acoustically actuated printer such as is shown in U.S. Pat. No. 5,565,113 by Hadimioglu et al. titled "Lithographically Defined Ejection Units" and incorporated by reference hereinabove. The droplet emitter 10 has a base substrate 12 with transducers 16 on one surface and acoustic lenses 14 on an opposite surface. Attached to the same side of the base substrate 12 as the acoustic lenses is a top support 18 with channels, defined by sidewalls 20, which hold a flowing liquid 22. Supported by the top support 18 is a capping structure 26 with arrays 24 of apertures 30. The transducers 16, acoustic lenses 14, and apertures 30 are all axially aligned such that an acoustic wave produced by a single transducer 16 will be focussed by its aligned acoustic lens 14 at approximately a free surface 28 of the liquid 22 in its aligned aperture. When sufficient power is obtained, a droplet is emitted.
FIG. 2 shows a perspective view of the droplet emitter 10 shown in FIG. 1. The arrays 24 of apertures 30 can be clearly seen on the capping structure 26. Each array 24 has a width W and a length L where the length L of the array 24 is the larger of the two dimensions. Arrow Lf shows the flow direction of the flowing liquid 22 through the top support 18, which is in the direction of the length L and orthogonal to the width W of the channels formed by sidewalls 20 and is along a length L of the arrays 24. This is due to the channels formed by sidewalls 20 being constructed such that the flowing liquid 22 flows in the direction of the length L of the each array. This configuration has many advantages. It is compact and allows the precisely aligned production of multiple arrays 24 of apertures 30 where each array is associated with a liquid having different properties. For instance, to enable a color printing application each array might be associated with a different colored ink. Furthermore, this configuration is easy to set up and attach to an ink pumping system. However, the pressure loss of the liquid 22 along the channel length L is dependent on the cross sectional area defined by sidewalls 20 and the channel length L. As the channel length L increases, the pressure loss along the flow direction increases. The portion of the pressure loss due to flow frictional losses is largely dependent upon and limited by the height h of the channel.
This pressure loss along the flow direction can become large and results in a limited flow rate. The pressure loss and the limited flow rate impacts the performance of the droplet emitter 10 by limiting the droplet emission rates possible in three ways. Firstly, the pressure loss will change the level of the free surface 28 of the flowing liquid in the apertures along the length L. At the very least, different liquid levels will contribute to focussing errors of the acoustic energy focussed by the acoustic lenses 14 and result in emitted droplets not landing in their target spots. For example, using a configuration of the type shown in FIGS. 1 and 2, with a length L of 1.7 inches and a flowing liquid having a viscosity of less than 1.3 centepoise, a flow rate which exceeds 10 ml per minute will exceed the focussing level tolerance of the acoustic lenses because the difference in meniscus position between the first and last emitter will exceed 5 microns. If the flow rate exceeds 35 ml per minute, the system can not sustain the free surface 28 of the flowing liquid 22 in the apertures 30. At these flow rates both simultaneous spilling and air bubble ingestion occurs.
Secondly, the slow flow rate will also mean that the flowing liquid 22 and the substrate 12 will heat up from the portion of the acoustic energy that is absorbed in the flowing liquid 22 and the substrate 12 which is not transferred to the kinetic and surface energy of the ejected drops. The liquid can sustain temperature increases by only a few degrees centigrade before emitted droplets show drop misplacement on the receiving media. In a worst case scenario, the flowing liquid 22 can absorb enough energy to cause it to boil. The practical consequences of this are that either the array length L, and hence the droplet emitter length must be very short to allow for faster flow rates or that the emission speed must be kept very slow to prevent the liquid from absorbing excess energy and heating up to unacceptable levels.
Using the example given above, with a configuration as shown in FIGS. 1 and 2 and a length L of 1.7 inches running under a maximum emission rate with all emitters emitting at approximately 30 watts, the temperature difference between the first and last emitter is approximately between 39 degrees centigrade and 75 degrees centigrade. This temperature differential is clearly above the preferred range of just a few degrees centigrade and affects the accuracy of droplet placement quality greatly. To correct this issue either the flow rate of the flowing liquid must be increased or the emission rate must be greatly reduced so that less heat energy is generated in the base substrate 12 and the flowing liquid 22. However, using the design shown in FIGS. 1 and 2, increasing the flow rate of the flowing liquid 22 results in an unacceptable pressure loss and meniscus position variance as discussed above. Therefore, using the design shown in FIGS. 1 and 2, emission rates must be kept low to prevent excess heating of the flowing liquid 22 to achieve acceptable drop placement accuracy.
Thirdly, if the droplet emitter is emitting droplets at high emission rates, a greater volume of fluid will be lost to droplet emission than can be replaced by the slow flow rates. Again the practical consequences of this are that either the array length L, and hence the droplet emitter length must be very short to allow for faster flow rates or that the emission speed must be kept slow to allow sufficient replenishment times.
Therefore, it would be highly desirable if a droplet emitter 10 could be designed to maintain a substantially constant pressure along the emission portion of the liquid flow path and which also has a faster flow rate for a droplet emitter array of any arbitrary length L with a minimal rise of the liquid flow temperature at high emission speeds and has sufficient liquid replenishment rates.
Further advantages of the invention will become apparent as the following description proceeds.